A wide variety of manually operable devices have been suggested for use as keyswitches forming component parts of a keyboard. Various mechanical as well as magnetically operable switches are in current use. A form of keyswitch which is finding increased utilization is the capacitively operated switch such as the device described in U.S. Pat. No. 3,671,822 issued June 20, 1972, to Theodore M. Leno entitled "Variable Capacitive Apparatus". The referenced switch includes a pair of conductive plates spaced to form a capacitor. The separation of the plates and thus their mutual capacitance is altered in response to manual actuation of a keytop. In response to the capacitance change a closure signal is generated which is processed by an encoder converting the signal, which is scan position related, into another notation such as binary or ASCII. The keyswitches are electronically scanned at a high rate and means are often provided for producing a single control signal representative of an actuated key regardless of the number of times the actuated keyswitch is scanned.
With continued reference to such keyswitches, the capacitive value of the keyswitch is sensed and this measure is utilized to determine the state of the sampled keyswitch. The sensing circuitry is frequently implemented with TTL or MOS logic elements. Such logic elements exhibit varying characteristics as a result of aging, temperature, voltage variations and processing parameters; the results of which may adversely affect the reliability of the sensing circuitry. MOS logic elements are particularly susceptible to such conditions. The illustrated embodiment serves to sense the operative condition of a capacitive keyswitch so as to produce reliable operation relatively unaffected by manufacturing and environmental conditions to which the various components forming the apparatus are subjected.